The PM is included in stand-alone forums as a core function because stand-alone forums present the whole website. But wpForo is not a stand-alone forum, it's a WordPress plugin and the website is presented by WordPress with lots of plugins. PM is a social community feature which is more likely Social Membership feature rather than forum feature. So in WordPress PM becomes  unavoidable feature for Profile and Social Membership builder plugins like BuddyPress, etc...

As you may already know there is no any WordPress forum plugin which has PM feature. wpForo  doesn't have PM feature in core as well.

Also, wpForo is  a free plugin and it's not a small plugin, this is a large plugin with very hard development and support. Dozens of people work on this plugin and they should have some income to keep the plugin stable development and actively improve it. The price you pay for paid addon includes:

1. Payment for addon development
2. Premium Support for the paid addon you've purchased.
3. Little donation to the hard work developing wpForo core for free
4. Little donation to the hard work for wpForo support team, we support dozens of wpForo users per day. We answer and solve hundreds of issues per day through forums and emails.

I understand. It's a lot of work. Tanks for the explanation.

It's actually a very small price to pay, especially when you consider the other options out there.